Another good way to get around is to rent a bicycle. This bicycle rental place is just around the corner from Humboldt-University:
Fahrradstation HU Berlin
Dorotheenstr. 30
10117 Berlin
Tel. +49 30 / 28 38 48 48
Fax +49 30 / 28 38 48 77 http://www.fahrradstation.com/fahrradstation.php/aid/106/cat/77/1
Prices are about € 50 per week (10% discount if you book online).